The present invention relates to a method for producing a Group III nitride compound semiconductor device.
A Group III nitride compound semiconductor device such as a blue light-emitting device or the like was obtained in such a manner that a buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) was grown on a sapphire substrate by a metal organic chemical vapor deposition method (referred to as xe2x80x9cMOCVD methodxe2x80x9d in this description) and a Group III nitride compound semiconductor layer was further grown thereon by an MOCVD method likewise.
In the MOCVD method, an ammonia gas and gases of Group III alkyl compounds such as trimethyl aluminum (TMA), trimethyl gallium (TMG) and trimethyl indium (TMI) were supplied onto a substrate heated at a suitable temperature, so that the gases were thermally decomposed to thereby form a film of desired crystal on the substrate. On this occasion, metalorganic materials such as TMA, etc. constituting raw material gases for the buffer layer were so expensive that they became a factor of causing increase of the cost of the Group III nitride compound semiconductor device.
If the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) is formed by another method than the MOCVD method, use of metalorganic materials such as TMA, TMG, etc. can be avoided. For example, Examined Japanese Patent Publication No. Hei. 5-86646 has made a proposal including the steps of: forming a buffer layer by a high-frequency sputtering method; heating a substrate (at a temperature of 800 to 1000xc2x0 C.) in an ammonia gas-containing atmosphere (of ammonia and nitrogen according to an embodiment); and supplying Group III metalorganic materials onto the heated substrate to thermally decompose the Group III metalorganic materials to perform vapor growth of a film of nitride thereof to thereby grow AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) of the same composition as the buffer layer on the buffer layer. Raw materials for forming the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) by the high-frequency sputtering method are high-purity metallic aluminum and high-purity metallic gallium. While these raw materials are used as a target, a mixture gas of argon and nitrogen is used as a sputtering gas. In this case, all the raw materials are inexpensive. Hence, the cost of the device can be reduced compared with the case where the buffer layer is formed by the MOCVD method using expensive organometals as raw materials.
The present inventors had tried the method disclosed in Examined Japanese Patent Publication No. Hei. 5-86646. As a result, the crystallinity of the Group III nitride compound semiconductor layer formed by the MOCVD method on the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) formed by the ratio-frequency sputtering method could not satisfy the present inventors"" requirement. That is, the crystallinity of the Group III nitride compound semiconductor layer obtained by the tried method is inferior to the crystallinity of the Group III nitride compound semiconductor layer formed by the MOCVD method on the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) formed by the MOCVD method.
The present inventors have made investigation again and again to improve the crystallinity of the Group III nitride compound semiconductor layer obtained by the sputtering method. As a result, the present inventors obtain a finding to define preferred conditions for forming the Group III nitride compound semiconductor layer on the substrate by the sputtering method. That is, a method for producing a Group III nitride compound semiconductor device according to the present invention is provided as follows.
A method of producing a Group III nitride compound semiconductor device, wherein an initial voltage of a sputtering apparatus is selected to be not higher than 110% of a sputtering voltage when a first Group III nitride compound semiconductor layer is formed on a substrate by a sputtering method.
In the method of producing a Group III nitride compound semiconductor device according to the present invention, the initial voltage is restrained to be not higher than 110% of the sputtering voltage when the first Group III nitride compound semiconductor layer is grown on the substrate by the sputtering method. Hence, it is conceived that the growth of the first Group III nitride compound semiconductor is stabilized at the initial stage of growth so that the first Group III nitride compound semiconductor layer gets into a state approaching to single crystal at the initial stage of growth. As a result, the first Group III nitride compound semiconductor layer becomes excellent in crystallinity as a whole.
On the other hand, if the applied voltage at the initial stage is high, the growth rate at the initial stage becomes unstable. As a result, it is conceived that the first Group III nitride compound semiconductor layer grown on the substrate different in material therefrom is easily made polycrystalline or amorphous rather than monocrystalline. This has influence on the whole of the first Group III nitride compound semiconductor layer to reduce the crystallinity of the first Group III nitride compound semiconductor layer.